1. Field of the Invention
The present invention relates to a carrier, an electrophotographic developer using the carrier, core material particles for the carrier, a production method of the core material particles and an image forming method using the electrophotographic developer.
2. Description of the Related Art
Developing processes of electrophotography are divided into a so-called one-component developing process only using a toner as a main component, and a so-called two-component developing process using a developer that is a mixture of a toner and a carrier. In such two-component developing process, a carrier is used, and thus the developer used in the two-component developing process, or two-component developer, has a wider area frictionally charged to toner particles. In addition, the two-component developing process is more stable in charge property than the one-component developing process, is advantageous in providing high-quality images over a long period of time and has a high-ability of supplying a toner to areas to be developed. Therefore, the two-component developer has been widely used.
In recent years, developing a developing system that has the capability of developing a latent image precisely has become important in order to correspond to increasing demands for higher resolution and better highlight reproducibility and to wider colorization. Thus, there have been various proposals from the aspect of both process conditions and developers including toners and carriers. From the aspect of the process, minimizing a developing gap, thinning a photoconductor and reducing a writing beam spot diameter are effective ways to correspond to such demands, but these ways increase the production cost and still have an unsolved big issue in, for example, reliability.
That using a small diameter toner in a developer can drastically improve the dot reproducibility is commonly known, however, that developer still has unsolved disadvantages in, for example, occurrence of background smears and inadequacy of image density. And resins with a low flexibility point have been used for small diameter full color toners in order to obtain sufficient color tones, however, using the resins will cause an increase of carrier spent, deterioration of a developer, toner scattering and background smears to the full color toners compared with a black toner.
There have been many proposals on using small diameter carriers. For example, Japanese Patent Application Laid-Open (JP-A) No. 58-144839 proposes a magnetic carrier that is composed of ferrite particles having spinel structures and an average particle diameter of less than 30 μm. This magnetic carrier is, however, not coated with resin and is used under a low developing electric field applied thereon, and has disadvantages in that the carrier has poor developing ability and, because the carrier is not coated with resin, has a short operating life.
Furthermore, Japanese Patent Application Publication (JP-B) No. 3029180 proposes an electrophotographic carrier having carrier particles with a 50% average particle diameter (D50) in the range of 15 μm to 45 μm, and containing particles smaller than 22 μm in diameter in the range of 1% to 20%, smaller than 16 μm in the range of 3% or less, 62 μm or larger in the range of 2% to 15%, and 88 μm or larger in the range of 2% or less, and wherein the specific surface area, S1, of the carrier which is determined by an air permeability method and the specific surface area, S2, of the carrier which is calculated by the equation, S2={(6/ρ)D50}104 (ρ represents a specific gravity of the carrier), satisfy the condition, 1.2<=S1/S2<=2.0.
Those small diameter carriers are known to have following advantages;that is, (1) Because of wide surface area per unit volume, toner particles will be given sufficient frictional electrification, and thus occurrences of low charged toner particles and oppositely charged toner particles can be suppressed. And as a result, occurrence of background smears and amount of dust and occurrence of toner blur around toner dots can be suppressed, and thereby high dot reproducibility can be achieved. (2) The average charge amount of the toner particles will be lowered because the carrier has a large surface area per unit volume and less occurrence of background smear, providing sufficient image densities. Therefore, the small diameter carrier particles can cover the shortcomings of using the small diameter toner particles, and are particularly advantageous in taking advantage of the toner particles. (3) The small diameter carrier particles can form a fine magnetic brush and have less tendency of causing blush smears.
Magnetic binding force of such smaller carrier particles, however, dramatically decreases proportionally with the cube of the decrease in the particle diameter, causing many carrier adhesions wherein carrier particles adsorb in a form of cut-off magnetic brush. As a result, such conventional small diameter carriers cause flaws of a photoconductor/fixing roller, and have big issue in practical use.
From a study on carrier particles adhering to a photoconductor, that smaller diameter carrier particles occupy a much greater portion among these adhering carrier particles than larger diameter carrier particles was found. Thus, there have been various proposals on classification steps to obtain carriers having a minimized particle diameter distribution. Among those suggested methods, a classification step using a sieve can provide carriers having a narrow particle diameter distribution and efficiently produce carriers having required particle diameters compared with a centrifugal force method and an air classification method. However, it is commonly known that the sieve classification method has difficulties in producing smaller carrier particles with a narrow particle diameter distribution because of the reduction in the mass per particle. Furthermore, smaller diameter carriers tend to have higher friction between particles and cause an increase in developing sleeve driving torque, resulting in scratching a surface of the sleeve and causing more toner fixations. That increases in friction and torque cause fluctuations in supply amount of developer to the sleeve and in the image density. Carrier particles having small diameters and BET specific surface areas have characteristic that, because of the smooth surface thereof, the carrier particles have small friction between which and require less developing sleeve driving torque, and thereby occurrences of scratching and toner fixation to the sleeve surface are prevented, resulting in less fluctuations in supply amount of developer to the sleeve and in the image density. Such carries having small diameters and BET specific surface areas, however, because of the smooth surface, or higher sphericity in the shape of the carrier particles, tend to be stuck in opens of the sieve, and thus obtaining the carrier particles from the sieve classification method has been particularly difficult. For that reason, obtaining the carries having small diameters and BET specific surface areas (or composed of core material particles having smooth surfaces) and the narrow particle diameter distribution had not been realized.
One proposed method, such as one disclosed in JP-A No. 2001-209215, includes transmitting supersonic vibrations to a metal screen of a classification machine, giving vertical acceleration to particles to thereby efficiently obtain particles 22 μm in diameters or smaller with a narrow particle diameter distribution in order to solve the forementioned problems and to obtain a carrier for an electrophotographic developer that can provide high image quality and high durability, cause less occurrences of carrier adhesions and have the weight average particle diameter (Dw) of from 25 μm to 45 μm, the content of particles 44 μm in diameter or smaller in the range of 70% or more, the content of particles 1.30 μm in diameter or smaller in the range of 7% or less, and the ratio of Dw to the number average particle diameter (Dp) ranging from 1.00 to 1.
This method can efficiently pass small diameter particles through the mesh because that virtical acceleration thereto applied substantially moves the particles as if small particles were having a large mass, or having a large true specific gravity. JP-A No. 2001-209215 further discloses using an ultrasonic transducer equipped with a resonant ring to improve the efficiency of the sieve method. However, when the classification machine uses a mesh having small openings, since the mesh is made with thin material and thus strength thereof is weak (as the mesh material is made with thin threads), a part of the edge of the mesh can easily broken due to the weight of the carrier particles after being used for a long time, resulting in that unclassified fine particles are mixed into the classified carrier particles, increasing the content of the fine particles. Furthermore, maintaining the classification performance over classification processes for particles having small particle diameters and BET specific surface areas is difficult even when a vibrating screen using the ultrasonic transducer equipped with the resonant ring is in use. That is because of occurrence of mesh clogging that was particularly big issue. Particles having small BET specific surface areas increase contact areas of the particles and the mesh threads and resistance of the particles to pass through the mesh, resulting in frequent occurrences of mesh clogging. Smaller diameter particles have higher tendency to cause the mesh clogging. When the mesh is clogged, as the carrier particles hide among the openings, it is quite difficult to remove the carrier particles, requiring an exchange of the mesh.
While some meshes are woven with resin threads, stainless steel is usually used therefor. That is because the resin threads has a small stiffness, and thus ultrasonic sound cannot be effectively transmitted to the mesh and classfication may be prevented at all. On the other hand, production costs of a stainless steel mesh having small openings are extremely high, resulting in higher carrier production costs.